1. Field of the Invention
The present invention relates to a developing device for applying an alternating voltage superimposed on a direct current voltage to a developer bearing member to thereby develop an electrostatic latent image formed on an electrostatic latent image bearing member with toner, and an image forming apparatus including the same.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a development method has been employed in which the surface of an electrostatic latent image bearing member (for example, a photoreceptor) is charged and an image is exposed to the charged region to from an electrostatic latent image, and the electrostatic latent image is developed so as to be made visible (developing).
As the development method, a development method has been commonly used in which, using one-component developer containing toner or two-component developer containing carrier and toner, by frictionally charging the toner so that the toner is attracted with an electrostatic force of an electrostatic latent image on the surface of the electrostatic latent image bearing member, the electrostatic latent image is developed to thereby form a toner image.
For example, when two-component developer is used, a method has been employed, in which a magnetic brush by carrier is formed on a developer bearing member (for example, a developing roller) in a developing device, and an electrostatic latent image is developed while applying a bias voltage between the developer bearing member and an electrostatic latent image bearing member.
Moreover, whether one-component or two-component developer is used, there is a case where development is performed using toner that is charged with a polarity opposite to a surface potential of the charged electrostatic latent image bearing member, or a case where reversal development is performed using toner that is charged with a polarity the same as the surface potential of the charged electrostatic latent image bearing member.
In addition, there is also a case where an electrostatic latent image that is formed on the electrostatic latent image bearing member is developed with the toner by applying an oscillating bias voltage between the developer bearing member and the electrostatic latent image bearing member. In this oscillating bias voltage, a development-side electrical potential that can apply a force to the charged toner in the direction from the developer bearing member toward the electrostatic latent image bearing member and an opposite development-side electrical potential that can apply a force to the toner in the direction from the electrostatic latent image bearing member to the developer bearing member alternate with each other, and for example, a rectangular wave is commonly used whose ratio (duty ratio) of the application time during which the development-side electrical potential is applied to the application time of a cycle during which the development-side electrical potential and the opposite development-side electrical potential are applied is 50%.
Incidentally, in such a conventional development method, it is desirable that the charge amount of the toner is increased to obtain smooth image quality with little roughness. However, for example, when two-component developer is used, the electrostatic force between carrier and toner is in proportion to the square of the charge amount, thus, when the charge amount of the toner is increased, a rate that development is performed with the carrier separated from the toner decreases. Accordingly, the utilization efficiency of the toner consequently deteriorates and the image density is reduced.
In order to increase the image density, an oscillation amplitude voltage Vpp (peak-to-peak voltage) of the oscillating bias voltage may be increased. However, when the Vpp is increased, an electric field in the direction where the toner returns from the electrostatic latent image bearing member to the developer bearing member is strengthened, thus a toner image that has been attached to the electrostatic latent image bearing member once is peeled off and dot is not added completely. That is, so-called dot reproducibility tends to deteriorate.
As a developing device that achieves both improved image density and dot reproducibility, Japanese Unexamined Patent Publication JP-A 2000-347507 discloses a developing device for reducing a peak-to-peak voltage of an oscillating bias voltage periodically.
In a commonly used bias voltage, the Vpp is constant and not changed, however, compared to this, when the Vpp of the oscillating bias voltage is reduced periodically like the developing device described in JP-A 2000-347507, it is possible to make the image density higher and the dot reproducibility is also improved slightly.
However, when the Vpp is increased even slightly, dot reproducibility deteriorates markedly. This is because, although the separation amount of toner from carrier is the largest in a state where the maximum Vpp is obtained, the subsequent Vpp is reduced only slightly and a force is applied greatly in the direction of returning toner from the electrostatic latent image bearing member to the developer bearing member.